The production and use of sheet and bulk molding compounds are well known. Such molding compounds are based upon unsaturated polyester resin systems and are widely used in the production of automobile body parts. Usually such systems are based upon a combination of an unsaturated polyester resin contained in a monomer and a thermoplastic soluble in the monomer. These materials, in the form of a syrup, are blended with gelling agents, crosslinking catalysts, release agents and, optionally, colorants, inhibitors, and the like.
This blend of materials is produced in sheet form by laying down a first layer of the molding composition on a first layer of polyethylene film or the equivalent thereof, laying on this first layer of the molding composition a filler, for example, chopped glass fibers in an amount up to about 30 percent by weight of the total composition, and laying thereover a second layer of the molding composition. The two layers of molding composition with the filler sandwiched therebetween are then topped with a second sheet of polyethylene film and the composite is stored. During storage, the composite thickens to a viscosity at which the polyethylene film can be stripped from the composite with no substantial adherence of the molding composition to the film. At this viscosity, upon molding, the molding composition carries the filler with it to produce a substantially uniform distribution of the filler throughout the mold.
This thickening, or gelling process is vital to the production of the molding composition. The thickening must be slow enough to allow wet-out and impregnation of the glass reinforcement. However, the thickening must be fast enough to allow the handling required by molding operations as soon as possible in order to keep the amount of sheet in storage at a low inventory level. While the thickening must give a viscosity at molding temperatures low enough to permit sufficient flow to fill out the mold at reasonable pressure, the thickening must give a viscosity high enough to carry the glass filler or other reinforcement along as the composition flows in the mold. Ideally, however, and this is the aspect where prior art molding compositions have proved unsatisfactory, thickening should stop or the rate of thickening should substantially decrease in a viscosity range which permits molding at low perssures, that is, at pressures within the range of from about 30 to about 250 pounds per square inch in contrast to the 500 to 1500 pounds per square inch required by prior art molding compounds.
Prior art molding compounds are thickened often by the incorporation of thickening agents such as calcium hydroxide. The use of chemical thickening agents sometimes requires storage of the composite for several days before it can be used. The use of chemical thickeners also is plagued by the sensitivity of the chemical thickening reaction to temperature, moisture, and the particular chemical composition of the resin being cured. When chemical thickeners are used, control of the ultimate viscosity of the sheet molding composition is also limited.
An object of this invention accordingly is to provide a method of thickening sheets of molding composition composites which eliminates the use of chemical thickening agents. Another object of the invention is to provide a method of thickening wherein the degree of thickening can readily be controlled. These and other objects of the invention will be readily understood from the following disclosure of the invention.